The first image illustrates the land impact of all the prims + the Harding window when all are selected, but not linked. Since the Harding window has land impact 2, the object tool shows 7 objects selected and the land impact is 8.
Normally, one would think that linking all the prims and the window would result in the same land impact, but currently, that is not what happens in Second Life, when prims and meshes have a default physics shape type. There are four basic principles you should know in this case:
1. Linking meshes to simple cube prims will help land impact decrease.
2. Linking meshes to complex prims, such as spheres, torus, tortured prims, sculpts or even scripted prims will help land impact increase. 3. If a mesh is scripted, chances are, land impact will be higher than the same unscripted version of that mesh. The more complex the script, chances are, the greater impact it may have on the land impact.
4. If the scripted mesh can't be the root prim, make sure it will only be linked to a simple prim and not the overall structure you're working on.

The picture above shows that the Harding window has land impact of 2 prims and is surrounded by 5 simple prims + 1 tortured prim. So what happens if we decide to link everything? Since we linked to a tortured prim, the land impact will "explode" to 11. Which is why we must be careful linking meshes to complex prims!

So what can we do? We can take advantage of the fact that meshes do decrease land impact when connected to prims with a physics shape type set to convex hull.

When physics shape types of meshes and prims are convex hull

We saw how infective it can be to link prims and meshes without consideration for physics shape types. So now, with all the prims and the window linked, lets edit our object and make sure all the surrounding prims of the window have a convex hull shape. Check the feature tab and click the convex hull shape option.

The mesh and prims linked have now a land impact of 4!

What if the land impact still "explodes"?

Sometimes, even if you set all the prims and meshes with a convex hull shape type, you still end up with a "bloated" mesh, specially if there are complex shapes in the equation (could be a sphere, a sculpt, a torus or a particularly tortured prim). Because of this, you may always try:

1- unlinking the complex shape from the rest of the structure and remain all the other prims linked.2- using 'None' physics setting on some complex prims in the structure to take them out of the equation.3- experiment yourself, and try different combinations with linking. You may be pleasantly surprised.

If you have any other questions, feel free to contact me. If you found this post useful and need to explain how meshes can be linked to others, feel free to send them the link to this post.